Regulator Noise Compensation

Low drop-out voltage regulators must often achieve tight supply noise rejection (PSRR) over a wide spectrum of operation. Good PSRR can be obtained within the bandwidth of the regulator feedback loop. However, PSRR quickly degrades outside the loop bandwidth, reaching a peak, before dropping again, for example, because of the load capacitance.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the implementations and are not necessarily drawn to scale. The edges of features drawn in the figures do not necessarily indicate the termination of the extent of the feature.

The making and using of various implementations are discussed in detail below. It should be appreciated, however, that the various implementations described herein are applicable in a wide variety of specific contexts. The specific implementations discussed are merely illustrative of specific ways to make and use various implementations, and should not be construed in a limited scope. Unless specified otherwise, the expressions “about”, “around”, “approximately”, “substantially”, and other unspecifying terms signify values within expected variation resulting from, for example, design, measurement, and/or manufacturing tolerances of the specified value, as expected by those of skill in the art. Unless specified otherwise, the expressions “equal”, “similar”, “proportional”, or other relational terms are understood to signify or include that the relation is substantially equal, substantially similar, substantially proportional, etc.

Low drop-out voltage regulators are expected to achieve high supply noise rejection (PSRR) over their full spectrum of operation. High PSRR can generally be obtained within the bandwidth of the regulator feedback loop through regular regulator regulation operation. However, PSRR can quickly degrade outside the feedback loop bandwidth, reaching a peak, and dropping again because of, for example, other circuit characteristics, such as the load capacitance. The PSRR peak may be affected, for example, by such circuit features as the size of the power transistor, parasitics on controlling nodes, and the regulator architecture, all things usually decided and engineered to meet other critical requirements. Thus, small improvements in peak PSRR often come as expensive trade-offs with other important performance objectives and specifications.

PSRR in voltage regulators is usually very good within the bandwidth of the regulator feedback loop (in-band PSRR) but PSRR quickly degrades outside that bandwidth and reaches a peak that can be several tens of dB worse. Such poor peak PSRR translates to large residual noise on the regulated supply which often causes serious degradation in system performance. Improving PSRR often entails the use of large decoupling capacitances with considerable area costs and limitations on the regulator dynamics or expensive trade off with other critical parameters.

The supply noise compensation circuit aspects discussed herein drastically improve the peak PSRR without modifying the power transistor or the regulator loop dynamics. Therefore, no circuit performance tradeoffs with other objectives and specification are required.

In some implementations, a filter senses out-of-band supply noise in the unregulated supply, which is amplified, and then injected, as a current in opposite phase, to the regulator output node. As a result, out-of-band supply noise which would otherwise affect the regulated supply voltage, is effectively or at least partially compensated for. In some implementations, the circuit also functions as a current bleeder, thus reducing or eliminating inclusion of a separate current bleeder circuit. Accordingly, in some implementations, the circuit improves PSRR in a way that is particularly efficient for area and power.

The circuit here described significantly improves peak PSRR in low-drop-out voltage regulators. This result is achieved with minimum area overhead, little or no trade off with other important specifications and, in some implementations, without modification to the regulator power transistor. In some implementations, the dynamics of the regulator feedback loop are not affected or are not significantly affected.

illustrates a regulator circuitcomprising a regulatorcoupled with a noise compensation circuit, which collectively provide a low noise high PSRR regulated voltage for a loadaccording to some implementations.

Regulatorreceives a reference voltage at node Vref and receives an unregulated supply voltage. In some implementations, the “unregulated” supply voltage is unregulated or is less regulated with respect to the regulated supply voltage generated by regulator. Accordingly, in some implementations, the unregulated supply voltage has some voltage regulation. Based on the unregulated supply voltage and the reference voltage, regulatorgenerates a regulated supply voltage. The type of regulator and the architecture or topology of regulatoris not limited. For example, regulatormay be or include features similar or identical to any of a number of low dropout (LDO) regulator circuits. Regulatormay be or include features similar or identical to any of a number of other regulator circuits.

Noise compensation circuitreceives the unregulated supply voltage, and generates a noise compensation signal which is coupled onto the regulated supply voltage generated by regulator. In some implementations, noise compensation circuitsenses or detects noise on the unregulated supply voltage. In some implementations, noise compensation circuitsenses or detects high-frequency noise, or noise which is out of band of regulator.

Furthermore, noise compensation circuitmay be configured to generate the noise compensation signal based on the sensed or detected noise signal. As a result, the noise compensation signal has properties similar to the noise sensed or detected on the unregulated supply voltage, such that the noise compensation signal has an effect on the regulated supply voltage which is equal to, substantially equal to, or about equal to, and opposite of the effect on the regulated supply voltage of the noise sensed or detected on the unregulated supply voltage. Accordingly, the noise cancellation signal at least partially compensates for the noise of the unregulated voltage.

Consequently, the noise compensation signal generated by regulatorgreatly improves PSRR in voltage regulator circuits without requiring large decoupling capacitances, thus saving important die area. Because noise compensation circuitoperates by sensing the unregulated supply and feeding the amplified, correcting signal directly to the regulator output node, noise compensation circuitdoes not affect or does not significantly affect the regulator feedback loop dynamics. In addition, noise compensation circuitmay be used with any regulator architecture.

In some implementations, noise compensation circuitalso provides a current bleeder function providing a minimum current load to regulator. Accordingly, by replacing a traditional current bleeder, in some implementations, noise compensation circuitdoes not require any additional current.

illustrates a noise compensation circuitaccording to some implementations. Noise compensation circuitincludes noise sensor circuit, noise amplification circuit, and noise injection circuit. In some implementations, noise compensation circuitis configured to sense noise from an unregulated supply voltage and to inject a corresponding canceling noise signal onto a regulated supply voltage, where the regulated supply voltage is generated, for example, by a regulator circuit based on the unregulated supply voltage.

Noise sensor circuitis configured to sense or detect noise in an unregulated supply voltage. In some implementations, noise sensor circuitis configured to sense or detect noise in a frequency bandwidth greater than a bandwidth of the regulator circuit which generates the regulated supply voltage. In some implementations, noise sensor circuitis configured to generate a noise signal based on the sensed or detected noise. Nonlimiting examples of noise sensor circuits are discussed elsewhere herein.

Noise amplification circuitis configured to receive the noise signal generated by noise sensor circuit. In addition, noise amplification circuitis configured to amplify the received noise signal to generate an amplified noise signal. Nonlimiting examples of noise amplification circuits are discussed elsewhere herein.

Noise injection circuitis configured to receive the amplified noise signal from noise amplification circuit. In addition, noise injection circuitis configured to generate a noise injection signal corresponding with the amplified noise signal, and to inject the noise injection signal onto the regulated supply voltage. Noise injection circuits are discussed elsewhere herein.

The noise sensor circuit, the noise amplification circuit, and the noise injection circuitare collectively configured to generate and inject a signal onto the regulated supply voltage, where the signal at least partially cancels or compensates for noise on the regulated supply voltage present as a result of the noise on the unregulated supply voltage.

Since the noise compensation circuitsenses and responds to the unregulated supply noise, no feedback loop is present. As a result, its dynamics can be made very fast to be effective even at very high noise frequencies.

Many voltage regulators use current bleeders, for example, to guarantee a minimum load current on the regulated voltage node and improve feedback loop stability of the regulator circuit. In some implementations, noise compensating circuitfunctions as a current bleeder circuit, thus, avoiding any additional current consumption overhead and minimizing the area footprint.

illustrates a noise compensation circuitaccording to some implementations. Noise compensation circuitincludes bias voltage generation circuit, noise amplification circuit, and transconductance stage. Noise compensation circuitincludes circuitry which performs the functions of noise sensor circuit, noise amplification circuit, and noise injection circuit. Noise compensation circuitand its components illustrate circuit concepts and principles which may be used in certain implementations. Numerous alternative implementations are contemplated.

Bias voltage generation circuitincludes transistor Mcoupled to the unregulated supply in a diode connected configuration. In this implementation, bias voltage generation circuitalso includes a ground-referenced low-pass filter comprising resistor Rand capacitor C.

Transistor Mgenerates a bias voltage at its gate based on the unregulated supply voltage and based on the current of current generator I. In addition, an input of the low-pass filter is connected to the gate of transistor M, and an output of the low-pass filter is connected to the gate of transistor Mof noise amplification circuit. Also, because the low-pass filter is referenced to ground, the transistor Mis effectively a common-gate stage, at least in frequency bands of interest.

Accordingly, from the unregulated supply point of view, this architecture results in a common-gate gain stage capable of sensing noise on the unregulated supply at frequencies higher than the pole set by the low pass filter. In some implementations, the pole is positioned at a frequency 1/X times the unity gain bandwidth of the regulator feedback loop, where X is the total gain of the noise compensation circuit. In some implementations, the bandwidth of the low pass filter is about equal to the frequency of minimum PSRR of regulator. In some implementations, the bandwidth of the low pass filter is about equal to and is less than the frequency of minimum PSRR of regulator.

As a result, the architecture provides a PMOS current mirror comprising transistors Mand M, where bias voltage generation circuitgenerates a low bandwidth bias voltage at the gate of transistor M. As a result, high-frequency noise on the unregulated supply voltage causes a noise current signal to be generated by transistor Mof noise amplification circuit, where the noise current signal is generated with a gain of the transconductance of transistor M(gm M)

Noise amplification circuitalso includes diode connected NMOS transistor M, forms an active load with resistor Rand capacitor C. Resistor Rand capacitor Cform a low-pass filtered gate bias voltage at the gate of transistor M. In some implementations, RCis about equal to RC. The architecture of noise amplification circuitprovides a high frequency transconductance gain of the noise current signal from transistor Mto generate a noise voltage signal at the drain of transistor Mwith a high frequency gain. In some implementations, the high frequency gain is equal to, substantially equal to, or about equal to the transconductance of transistor M(gm M) times the drain to source resistance of transistor M(Rds M) in parallel with the drain to source resistance of transistor M(Rds M).

Transconductance stagereceives the noise voltage signal and generates a current noise cancellation signal which is injected into the regulated supply voltage. The current noise cancellation signal is generated with an inverting gain equal to the transconductance of transistor M(−gm M).

Accordingly, noise compensation circuitgenerates a noise cancellation signal equal to the noise of the unregulated supply voltage times (gm M)×(Rds M∥Rds M∥R)×1/(gm M).

In some implementations, one or more of the gain factors of noise compensation circuitare programmable. For example, one or more transistors of noise compensation circuitmay include multiple segments or legs which may be selectively connected or disconnected to the circuit to program the gain of the noise compensation circuit. For example, in some implementations, a regulator circuit using noise compensation circuitundergoes a calibration sequence to determine a gain for noise compensation circuitwhich preferentially compensates for noise in the regulated supply voltage generated as a result of noise in the unregulated supply voltage.

Since the noise compensating circuit senses and responds to the unregulated supply noise, no feedback loop is present. As a result, its dynamics can be made very fast to be effective even at very high noise frequencies.

Many voltage regulators use current bleeders, for example, to guarantee a minimum load current on the regulated voltage node and improve feedback loop stability of the regulator circuit. Since noise compensating circuitfunctions as a current mirror at low frequencies, it is also effectively a current bleeder circuit, thus, in some implementations, avoiding any additional current consumption overhead and minimizing the area footprint.

In some implementations, noise compensation circuitis used with a regulator having a PMOS output transistor driving the regulated supply. A beneficial aspect of noise compensation circuitused with such a regulator is that the gain of the noise from the unregulated supply to the regulated supply from noise compensation circuittracks the noise from the unregulated supply to the regulated supply through the PMOS output transistor across process, voltage, and temperature variations.

illustrates a noise compensation circuitaccording to some implementations. Noise compensation circuitincludes bias voltage generation and noise coupling circuit, noise amplification circuit, and transconductance stage. Noise compensation circuitincludes circuitry which performs the functions of noise sensor circuit, noise amplification circuit, and noise injection circuit. Noise compensation circuitand its components illustrate circuit concepts and principles which may be used in certain implementations. Numerous alternative implementations are contemplated.

Bias voltage generation and noise coupling circuitincludes transistor Mcoupled to ground in a diode connected configuration. In this implementation, bias voltage generation and noise coupling circuitalso includes a low-pass filter referenced to the unregulated supply voltage, where the low-pass filter includes comprising resistor Rand capacitor C.

Transistor Mgenerates a bias voltage at its gate based on the current of current generator I. In addition, an input of the low-pass filter is connected to the gate of transistor M, and an output of the low-pass filter is connected to the gate of transistor Mof noise amplification circuit. In addition, because capacitor Cis connected to the unregulated supply voltage, high-frequency noise of the unregulated supply voltage is coupled to the output of the low-pass filter.

Accordingly, from the unregulated supply point of view, this architecture results in a common-gate gain stage capable of sensing noise on the unregulated supply at frequencies higher than the pole set by the low pass filter. In some implementations, the pole is positioned at a frequency 1/X times the unity gain bandwidth of the regulator feedback loop, where X is the total gain of the noise compensation circuit. In some implementations, the bandwidth of the low pass filter is about equal to the frequency of minimum PSRR of regulator. In some implementations, the bandwidth of the low pass filter is about equal to and is less than the frequency of minimum PSRR of regulator.

As a result, the architecture provides an NMOS current mirror comprising transistors Mand M, where bias voltage generation and noise coupling circuitgenerates a low bandwidth bias voltage at the gate of transistor Monto which noise of the unregulated supply voltage is coupled. As a result, high-frequency noise on the unregulated supply voltage causes a noise current signal to be generated by transistor Mof noise amplification circuit, where the noise current signal is generated with a gain of the transconductance of transistor M(gm M)

Noise amplification circuitalso includes diode connected PMOS transistor Mwhich forms a current mirror with PMOS transistor M. Consequently, the noise current signal is mirrored by PMOS transistors Mand Mto form a mirrored noise current signal. In some implementations, the current mirror provides a gain or amplification. We can call that gain gMM.

Noise amplification circuitalso includes diode connected NMOS transistor M, which forms an active load with resistor Rand capacitor C. Resistor Rand capacitor Cform a low-pass filtered gate bias voltage at the gate of transistor M. In some implementations, RCis about equal to RC. The architecture of noise amplification circuitprovides a high frequency gain of the noise current signal from transistor Mto generate a noise voltage signal at the drain of transistor Mwith a high frequency gain. In some implementations, the high frequency gain is equal to, substantially equal to, or about equal to the drain to source resistance of transistor M(Rds M) in parallel with the drain to source resistance of transistor M(Rds M).

Transconductance stagereceives the noise voltage signal and generates a current noise cancellation signal which is injected into the regulated supply voltage. The current noise cancellation signal is generated with an inverting gain equal to the transconductance of transistor M(−gm M).

Accordingly, noise compensation circuitgenerates a noise cancellation signal equal to the noise of the unregulated supply voltage times (gm M)×(gMM)×(Rds M∥Rds M∥R)×1/(gm M).

In some implementations, one or more of the gain factors of noise compensation circuitare programmable. For example, one or more transistors of noise compensation circuitmay include multiple segments or legs which may be selectively connected or disconnected to the circuit to program the gain of the noise compensation circuit. For example, in some implementations, a regulator circuit using noise compensation circuitundergoes a calibration sequence to determine a gain for noise compensation circuitwhich preferentially compensates for noise in the regulated supply voltage generated as a result of noise in the unregulated supply voltage.

Since the noise compensating circuit senses and responds to the unregulated supply noise, no feedback loop is present. Furthermore, since noise compensating circuitfunctions as a current mirror at low frequencies, it is also effectively a current bleeder circuit.

In some implementations, noise compensation circuitis used with a regulator having a NMOS output transistor driving the regulated supply. A beneficial aspect of noise compensation circuitused with such a regulator is that the gain of the noise from the unregulated supply to the regulated supply from noise compensation circuittracks the noise from the unregulated supply to the regulated supply through the NMOS output transistor across process, voltage, and temperature variations.

illustrates a regulator circuitcomprising a regulatorcoupled with a noise compensation circuit, which collectively provide a low noise high PSRR regulated voltage for a loadaccording to some implementations.

Regulatorreceives a reference voltage at node Vref and receives an unregulated supply voltage. Based on the unregulated supply voltage and the reference voltage, regulatorgenerates a regulated supply voltage. Regulatoris configured to generate a control signal which it internally uses to generate the regulated supply voltage. For example, regulatormay generate the control signal as part of a feedback loop causing the regulated supply voltage to have a desired voltage value.

Noise compensation circuitreceives the unregulated supply voltage and receives the control signal. In addition, noise compensation circuitgenerates a noise compensation signal based on the unregulated supply voltage and based on the received control signal. The noise compensation signal is coupled onto the regulated supply voltage generated by regulator. In some implementations, noise compensation circuitsenses or detects noise on the unregulated supply voltage. In some implementations, noise compensation circuitsenses or detects high-frequency noise, or noise which is out of band of regulator.

Furthermore, because the noise compensation signal is based on the sensed or detected noise signal, the noise compensation signal has properties similar to the noise sensed or detected on the unregulated supply voltage. Consequently, the noise compensation signal has an effect on the regulated supply voltage which is equal to, substantially equal to, or about equal to, and opposite of the effect on the regulated supply voltage of the noise sensed or detected on the unregulated supply voltage. Accordingly, the noise cancellation signal at least partially compensates for the noise of the unregulated voltage.

As a result, the noise compensation signal generated by regulatorgreatly improves PSRR in voltage regulator circuits without requiring large decoupling capacitances, thus saving important die area. Because noise compensation circuitoperates by sensing the unregulated supply and feeding the amplified, correcting signal directly to the regulator output node, in some implementations, noise compensation circuitdoes not affect or does not significantly affect the regulator feedback loop dynamics.